In the continuous sintering of mineral material, a layer of material is formed on a conveyor base in a sintering furnace, the layer being called herein a material bed. The material bed is conveyed by the conveyor base through the process zones of the sintering furnace, which have different temperatures. During the conveyance, gas is conducted through the conveyor base and the material bed when the material bed travels through the process zones.
From a last cooling zone, gas is recycled in a circulation gas duct to a drying zone that constitutes the first process zone. In the drying, the energy of the gas is used for heating the material bed and evaporating water. The gas cools and moistens, when it conveys heat to the evaporation. Exhaust gas conveys moisture away from the material bed. Because of the water transport, it is essential for the balance of the entire furnace that the gas flow through the bed remains constant.
The balance of materials and energy of the well-known sintering furnace is fairly complex due to three separate gas circulation processes from the cooling zones back to the drying, heating, and sintering zones. The process control is based on fixing the process parameters in the entire process, starting from raw material etc., to maintain the balance. The principle of controlling the sintering furnace is not to adjust individual zones at fixed points only, but to balance the temperatures in individual zones to acceptable ranges, so that the profile in the furnace remains in balance.
In prior art solutions, in practice, the drying temperature in the drying zone is controlled by regulating the volume flow of the gas flow that is conducted through the material bed, so that part of the hot gas flow of the circulation gas duct is conducted as a by-pass flow past the material bed and into an exhaust air blower. The regulation is carried out by a control valve that is arranged in the bypass gas duct, which when open, increases the flow and decreases the temperature, and when closed, decreases the flow and increases the temperature in the drying zone.
One problem with the existing system is that, in particular, if and when the change in the position of the control valve is great, it also influences the gas flow through the material bed in the drying zone and, thus, the process itself and the balance of the furnace.
An original and effective principle is to adjust the control valve manually because of the long response times of the control and because of the problem mentioned above. In practice, users have changed the adjustment of the control valve to be automatic, against the instructions. A problem with the automatic use is that it causes variations in the quality of the process and the product. If and when the control valve tries to keep the drying zone temperature at one standard value, the control valve easily fluctuates from side to side. At the same time, it also influences the gas flow through the material bed.